Polyurethanes (PU) are made water dispersible by using external emulsifiers. But they suffer a few disadvantages, as they require strong shear forces to disperse the polymer which results in coarse particles and poor dispersion stability. A unique method of dispersing PU in aqueous media is by structural modification, i.e., modifying hydrophobic PU backbone with built-in hydrophilic segments. Introduction of hydrophilic segments into PU backbone is achieved by incorporating non-ionic polyether segments. Reference may be made to Noll, et. al (U.S. Pat. No. 4,190,566), who used Polyethylene glycol as the hydrophilic segment. The dispersions obtained using polyethylene glycol have poor storage stability and film properties. Hydrophilic segments can also be introduced by incorporating ionic groups which can be cationic, as reported by Bechara, et al, (U.S. Pat. No. 6,339,125) or anionic, as reported by Wolfe, Jr., et al, (U.S. Pat. No. 4,183,836). In these methods the ionic groups are chain extenders which are incorporated at the chain extension stage. Hence this restricts the choice of the monomers to diamino/dihydroxy compounds bearing potential ionic centers. Since acrylics are used in leather finishing acrylics bearing ionic group can also be introduced into PU chain to impart hydrophilicity. To effect this PU backbone should possess initiating groups. PU containing iniferter groups can be used to incorporate acrylics. Polyurethane dispersions s how good abrasion resistance, soil resistance, low temperature flexibility and have excellent gloss. Also, they have good film adhesion, hence find application in leather coating. But polyurethanes are generally expensive.
Acrylic resins which were introduced in the 20th century in leather finishing due to their better light fastness, film adhesion, flexibility, resistance to water spotting and wet and dry rub fastness, became commercially predominant in leather finishing. An important feature about acrylics is their ability to bind a large amount of pigments. Hence a lower quality hides can be converted into good leather by the application of several coats of high covering pigments. But the drawbacks of all acrylics are their poor scuff resistance, wet rub fastness and low toughness. Moreover their thermoplastic nature leads to print retention but imparts tack at high temperatures. Unmodified acrylics give gripping touch to leather.
Copolymers can be prepared by conventional free radical polymerizations. The vinyl terminated polyurethanes are decomposed in the presence of free radical initiator and acrylic monomers to obtain PU acrylic copolymers. The main drawback of free radical polymerization is the lack of control over polymer molecular weight, architecture and end groups.
In water borne coatings, there is a greater sense in controlling the macroscopic structure for improved film properties. Control of macromolecular structure of monomer assemblies can be effectively carried out by Controlled polymerization.
In controlled polymerization several living radical polymerization techniques are being explored and iniferter technique is one of them. The advantages of iniferter method is that it is cheaper than other living polymerization technique, it can be easily prepared, the polymerization takes place at ambient temperature and does not require stringent conditions. More over, the polymerization of the vinyl monomers can be carried out in aqueous medium and hence the use of organic solvents can be avoided to a large extent.
The iniferters can be thermally dissociated to form radicals, which lead to further polymerization of the vinyl monomers. After the consumption of all vinyl monomers the iniferter forms dormant species by recombination, which can again be reactivated by increasing temperature. Hence almost all the properties like, hydrophilicity, hydrophobicity, mechanical properties, strength etc. can be altered, by using suitable monomers not only during the synthesis but even after preparing the final polymer.
In any other coating system, the properties of the material cannot be altered after the completion of the synthesis, due to the absence of reactive groups. Hence the modification of final properties is difficult even if it is required.